Lead-zinc-boron sealing glass compositions

ABSTRACT

Sealing compositions suitable for sealing semi-conductor packages and the like at temperatures below about 450°C. The compositions are mixtures of finely divided solder glass and an oxygen containing zinc material. The solder glasses are either lead-boron glasses or lead-zinc-boron glasses in which the zinc-lead mol ratio is below 1:2.

BACKGROUND OF THE INVENTION

Effort has been made in the past, without success, to provide theelectronic industry with a glass material for sealing semi-conductorceramic packages at the lowest possible temperature and with sufficientmechanical strength to maintain hermeticity during thermal shocks andother conditions as specified by the MIL-STD-883 specifications.

The technical requirements of an adequate sealing glass material aresevere. The material problem has arisen particularly with thedevelopment of the dual-in-line type ceramic packaging technology andits wide acceptance as a relatively inexpensive multilead hermeticpackage for integrated semi-conductor circuits. The glass seal must notonly bond two ceramic parts together but must also provide a stronghermetic seal with a substantially mismatched expansion metal lead framewithin the glass layer.

Similarly in the formation of cathode ray tubes, in order not to damagethe heat sensitive phosphor coating, metallic film shield or electricalcontact, glass parts must be sealed at the lowest possible temperature.

Heretofore successful sealing materials for the above applications havebeen made with glasses, known generically as solder glasses, containingmainly lead, zinc and boron oxides, mixed as a powder with an inert, lowexpansion ceramic powder such as beta-eucryptite, fused silica, orzirconium silicate to modify the internal structure of the glass oncethe seal has been formed by heat induced recrystallization. The presenceof a multitude of crystals within the glass layer, plus the presence ofa low expansion ceramic filler, prevents the propagation of surfacecracks through the glassy-polycrystalline seal which is subjected toconsiderable tensile stresses. These temperatures are too high to beused with a large proportion of semi-conductor devices, namely thoseknown in the semi-conductor industry as MOS (Metal Oxide Silicon), LIC(Linear Integrated Circuits) and CCD (Charge Coupled Devices) integratedcircuits, which are surface sensitive and prone to failure when heatedover about 430°C. Past attempts to lower the sealing temperaturesubstantially below 480°C have resulted in seals characterized by a lowhermeticity or poor resistance to thermal shock.

SUMMARY OF THE INVENTION

The low melting glass-filler compositions of this invention overcome theabove shortcomings of the prior art as they permit a glass seal to bemade at approximately 400°C. These glass seals tolerate imbedded metalleads with substantial expansion mismatch and maintain hermeticity evenafter a considerable number of liquid to liquid thermal shocks. This isa stress condition which even the higher temperature sealing glasses donot easily pass. Moreover, these glass-filler compositions providehermetic sealing for surface sensitive semi-conductor devices in ceramicpackages of large, as well as small, size.

Briefly, the invention comprises glass compositions containing lead,zinc, and boron oxides as the major components in which the zinc oxidecontent is made deliberately lower than the composition corresponding tothe 2PbO-ZnO-B₂ O₃ molar ratio (2:1:1 molar ratio). Such glasscompositions are very low melting and have softening points as low as300°C. These glass compositions are mixed in powder form with sufficientnon-inert zinc oxide containing filler powder to convert the glass to azinc oxide rich glass. This filler powder is used in amount such thatits available zinc oxide content is at least sufficient to permit thehot mixture to partially or completely crystallize forming a solidcrystalline phase in which the PbO-ZnO-B₂ O₃ molar ratio approaches2:1:1. On melting, the glass initially flows to form a seal and this isconverted from a lead-boron rich glass to a zinc oxide enriched glasspreferably corresponding to the molar ratio of 2 moles of lead oxide toone mole of zinc oxide and one mole of boron oxide as available ZnOdissolves. The amount of zinc oxide containing filler added to thesolder glass is sufficient to constitute 3 - 30% by weight of theglass-filler mixture. Very high mechanical strength is imparted by apolycrystalline structure containing well dispersed filler powder. Thepresence of the dispersed filler powder results from the employment ofzinc oxide contributing fillers in amounts exceeding the amount requiredto produce a PbO-ZnO-B₂ O₃ ratio of 2:1:1. Employment of filler inamounts exceeding 30% by weight of the glass-filler mixtures should beavoided.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Experience has shown that adequate mechanical strength is mismatchedglass-to-glass, glass-to-ceramic and ceramic-to-ceramic glass seals canbest be obtained with glasses which become recrystallized after the sealis formed. This is particularly true in semi-conductor packaging sealsin which metal leads are imbedded through the glass layer. A well knownexample is the type of ceramic packages generally known as thedual-in-line packages (CerDip) in which a complex etched or punchedmetal lead frame is imbedded between two glass layers supported by twoalumina ceramic parts. The thermal expansion mismatch between the glass,the metal frame and the ceramic parts is appreciable and of the order of15 to 20 × 10.sup.⁻⁷ per ° C.

The term recrystallization, or devitrification, is used here in itsconventional sense to mean a crystallization of glass wherein the glassis converted, or devitrified, to a crystalline phase, that is a rigidcrystalline skeleton which may be associated with a glassy matrix andwherein thermal and other material characteristics or properties, suchas viscosity and expansion, differ from those of the parent glass andare substantially determined by the crystalline phase.

The only glass materials which have been successful so far in being ableto maintain hermeticity in these packages of the order of 10.sup.⁻⁸ and10.sup.⁻⁹ std. cc/sec and mechanical strength after thermal cycling andthermal shock from about 150° to -65°C are recrystallizable glassescontaining lead, zinc and boron oxides mixed with an inert, refractorymetal oxide powder. These glasses recrystallize to near completeness toform a compound with the 2:2:1 lead oxide-zinc oxide-boron oxidestoichiometry.

The presence of a multitude of crystals within the glass seal, plus thepresence of a low expansion, inert refractory metal oxide filler,prevents the propagation of surface cracks through the resultingglassy-polycrystalline seal. It is of importance when preparing thistype of seal to choose a time-temperature cycle which insurescompleteness in the recrystallization of the glass. Improper orincomplete recrystallization severely weakens the mechanical strengthand the hermeticity of the seal.

Every attempt in the past at developing and commercializing sealingglasses with a lower melting point by, for example, reducing the zincoxide content, replacing part of the lead oxide by lead fluoride oradding other metal oxides or fluorides in the above glasses has producedseals which were substantially weaker and therefore undesirable.

                  Table 1                                                         ______________________________________                                                               Commercially Available                                                        Sealing Glass                                          Ratios by                                                                              2:1:1 compound                                                                              CV97, CV98*                                            Weight % Weight %      Weight %                                               ______________________________________                                        PbO      74.72         72                                                     ZnO      13.62         17                                                     B.sub.2 O.sub.3                                                                        11.7          10                                                     ______________________________________                                         *(marketed by Owens-Illinois under these designations)                   

Presently available sealing glasses are not suitable for the productionof hermetically sealed ceramic packages, particularly those with theCerDip geometry, with a sealing range appreciably below 480°C for areasonably short time, that is less than 20 minutes at peak temperature.

The new sealing glass-filler compositions according to this inventioncombine a low melting lead oxide rich solder glass with a non-inertfiller powder containing zinc oxide. We have noted that zinc oxide inpure or combined form has a tendency to dissolve in the glass when theglass is reheated, the rate of dissolution depending on the zinc oxidecompound. The filler powder is non-inert in the sense that it dissolvesin the solder glass and modifies the physical and chemical properties ofthat glass.

Heretofore dissolution of an added filler has been assumed to beundesirable because generally weak seals resulted from it, solublefiller materials have therefore been largely avoided. It has now beenfound that zinc oxide dissolves in and changes the composition of zincpoor PbO-ZnO-B₂ O₃ glasses to zinc richer glasses which canrecrystallize to the 2PbO-ZnO-B₂ O₃ crystalline phase and have unusuallyhigh strength.

Accordingly it is an object of this invention to provide a method formodifying a very low melting, high lead solder glass by the addition ofa sufficient amount of a filler material, which can be dissolved by theglass to provide enough missing zinc to produce a complete 2PbO-ZnO-B₂O₃ crystalline phase with practically no glassy phase left within theseal.

Accordingly it is another object to combine a zinc bearing filler with azinc-free lead borate glass or with a lead-zinc borate glass in whichthe zinc oxide content is lower than the required minimum of 13.6percent by weight (and therefore quite insufficient to insure completerecrystallization of the glass). These latter glasses possess acombination of very low melting points and high fluidity which promoteexcellent wetting of the parts to be sealed at a very low temperature.Once the seal is formed the zinc has begun dissolving into the glass,thus changing the glass composition toward the 2PbO-ZnO-B₂ O₃ ternaryphase diagram point and inducing recrystallization of the glass. Oncompletion of the sealing cycle any glass phase left in the seal hasdissolved enough zinc oxide and has fully recrystallized.

It is yet another object to describe a general method by which a glasspowder is mixed with a finely dispersed zinc containing filler. Onmelting the glass-filler mixture, the composition of the glass changesto a zinc rich glass which can recrystallize fully to form amechanically strong seal. Thus the concept of this invention is notlimited to low melting glass-zinc filler mixtures but can be extended toinclude any glass with little or no tendency to recrystallize withoutthe presence of zinc oxide.

Table 2 shows glass compositions, expressed in percent by weight, whichillustrate lead-zinc-borate and lead-borate type glasses which have beenfound particularly suitable in making seals when combined with a zincbearing ceramic powder and the combination is heated to temperatures ofthe order of 350° to 450°C.

                                      Table 2                                     __________________________________________________________________________    I      II III                                                                              IV V  VI VII                                                                              VIII                                                                             IX X  XI XII                                                                              XIII                                                                             XIV                                                                              XV                              __________________________________________________________________________    PbO 77.5                                                                             77.5                                                                             80.0                                                                             77.5                                                                             76.5                                                                             76.0                                                                             75.5                                                                             80.0                                                                             75.0                                                                             80.0                                                                             76.1                                                                             77.0                                                                             82.5                                                                             82.5                                                                             76                              ZnO 10.0                                                                             10.0                                                                             10.0                                                                             10.0                                                                             11.0                                                                             13.0                                                                             12.0                                                                             -- -- -- 9.5                                                                              10.0                                                                             5.0                                                                              3.0                                                                              11                              B.sub.2 O.sub.3                                                                   7.5                                                                              9.0                                                                              7.5                                                                              10.0                                                                             9.0                                                                              9.0                                                                              9.0                                                                              10.0                                                                             20.0                                                                             15.0                                                                             10.4                                                                             7.5                                                                              10.0                                                                             10.0                                                                             9                               SiO.sub.2                                                                         2.5                                                                              2.5                                                                              2.5                                                                              2.5                                                                              2.5                                                                              1.5                                                                              2.5                                                                              5.0                                                                              5.0                                                                              5.0                                                                              2.1                                                                              2.5                                                                              2.5                                                                              2.5                                                                              2                               SnO.sub.2          0.5                                                                              0.5                                                     P.sub.2 O.sub.5       0.5                                                     Al.sub.2 O.sub.3                                                                  2.5                                                                              1.0               5.0                                                  CuO                               1.9      1.0                                Bi.sub.2 O.sub.3                                                                              1.0                  0.5   1.0                                BaO                                           2                               __________________________________________________________________________

In Table 3 the molar compositions of the several glasses shown in Table2 are set forth.

                                      Table 3                                     __________________________________________________________________________    I       II III                                                                              IV  V   VI  VII VIII                                                                              IX X  XI  XII                                                                              XIII                                                                              XIV XV                     __________________________________________________________________________    PbO 0.35                                                                              0.35                                                                             0.36                                                                             0.35                                                                              0.34                                                                              0.34                                                                              0.34                                                                              0.36                                                                              0.34                                                                             0.36                                                                             0.34                                                                              0.34                                                                             0.37                                                                              0.37                                                                              0.34                   ZnO 0.12                                                                              0.12                                                                             0.12                                                                             0.12                                                                              0.14                                                                              0.16                                                                              0.15                                                                              --  -- -- 0.12                                                                              0.12                                                                             0.06                                                                              0.037                                                                             0.135                  B.sub.2 O.sub.3                                                                   0.11                                                                              0.13                                                                             0.11                                                                             0.145                                                                             0.13                                                                              0.13                                                                              0.13                                                                              0.145                                                                             0.28                                                                             0.22                                                                             0.15                                                                              0.11                                                                             0.145                                                                             0.145                                                                             0.13                   SiO.sub.2                                                                         0.04                                                                              0.04                                                                             0.04                                                                             0.04                                                                              0.04                                                                              0.025                                                                             0.04                                                                              0.08                                                                              0.08                                                                             0.08                                                                             0.03                                                                              0.04                                                                             0.04                                                                              0.04                                                                              0.03                   SnO.sub.2             0.003                                                                             0.003                                               P.sub.2 O.sub.5           .0015                                               Al.sub.2 O.sub.3                                                                  0.025                                                                             0.01                  0.05                                            CuO                                     0.024      0.013                      Bi.sub.2 O.sub.3  0.002                     0.03   0.002                      BaO                                                    0.013                  __________________________________________________________________________

While these glasses are particularly effective in making devitrifiedglass seals when finely divided and mixed with a zinc oxide contributingfiller, it will be appreciated that the invention may also be practicedwith other low zinc oxide glasses and under other conditions. Suitablelow zinc glasses are not only those shown in Tables 2 and 3, but includethose containing 75-90% by weight of lead oxide, 10-25% by weight boronoxide and 0-13% by weight of zinc oxide. The glass selected for making agiven seal will depend on various factors including the properties ofthe materials being sealed, such as their expansion characteristics andsoftening point, the specified sealing conditions, particularly thetemperature at which the seal is to be made or the permissible range oftemperatures, and somewhat on the nature of the seal itself. With thesefactors established, one skilled in the glass art can then select orcompound a semi-stable sealing glass material and determine thesuitability of the glass material with the aid of sealing tests such asdescribed subsequently.

The non-inert zinc oxide containing or contributing materials (simple orcomplex compounds, glasses or recrystallized glasses) which can be addedin the form of a powder, either singly or in any combinations, to theabove described low melting glasses in the range of 3.0 to 30% byweight, include the following: zinc aluminate, zinc borate, zinccarbonate, zinc chromate, zinc dichromate, zinc ferrate, zincfluosilicate, zinc gallate, zinc hydroxide, zinc permanganate, zincnitrate, zinc oxide, zinc orthophosphate, zinc aluminophosphate, zincpyrophosphate, zinc silicate, zinc orthosilicate, zinc titanate, zinczirconate, zinc stannate, zinc zirconium spinel, and zinc zirconiumsilicate.

Successful practice of the present invention requires a soft sealingglass. That is to say the glass must behave like a stable, soft sealingglass until a good seal is effected. A good seal between parts requiresthat the sealing glass be relatively soft and fluid at the sealingtemperature so that it can flow over and completely wet the sealingsurfaces of the preformed parts as well as completely fill the spacebetween such surfaces. If the glass is too stiff, reentrant angles,folds and the like occur and the seal is mechanically and thermallyweak. It is apparent then that the glass must be one which does notappreciably devitrify either during melting or in reheating prior to theformation of a seal.

It is quite desirable, however, that the sealing glassfiller materialdevitrify as rapidly as possible once a proper seal is formed.Preferably glass devitrification is initiated at the selected sealingtemperature thus permitting the assembly to be raised to the sealingtemperature and then held at that temperature for a short time, that isof the order of a few minutes to a quarter of an hour or so, whiledevitrification of the glass occurs.

For present purposes the expansion of the parent sealing glass is ofrelatively little importance, since the physical properties of theglass-filler mixture with the glass in its devitrified state determinethe amount and nature of the stress developed in the seal and theseproperties tend to be quite different from those of the parent glass.While physical properties of devitrified glass-filler mixtures,particularly expansion data can be measured by conventional methods, itis more convenient to rely on mechanical strength and hermeticity dataof the finished product. In the case of semi-conductor ceramic packagesthe MIL-STD-883 (issued by the Department of Defense on methods andprocedures for testing mecroelectronic devices and their packages,including basic environmental tests) testing conditions should be used.

Accordingly, the glass seals made pursuant to this invention not onlymeet, but substantially exceed the more demanding test conditions ofthermal shock (method 1011 -- Condition C), thermal cycling (method1010), corrosion (method 1009), high temperature storage (method 1008),hermeticity (method 1014) and high humidity (method 1004). These methodsare described in Code FSC 5962 published by the U.S. Department ofDefense, May 1, 1968.

In preparing a sealing glass-ceramic material for the present purposeconsiderable care should be taken to avoid contamination of the glassduring the mixing and smelting of the glass forming components and toinsure a uniform composition throughout the melt. After the glass meltis cooled, it is reduced to a powdered form preferably beingsufficiently fine to pass a standard 300 mesh screen. The powdered glassis thoroughly mixed with the desired zinc bearing filler which ispresent in quantities to constitute a 3.0 to 30 % by weight of themixture and preferably also fine enough to pass through a 300 meshscreen. The glass-filler mixture may then be mixed with a conventionalorganic binder and vehicle to form a suspension or slurry forapplication to a sealing surface. Any organic material used should becapable of completely burning out or volatilizing well below the sealingtemperature of the sealing material. A solution of 1-3% ethyl celluloseT-200 in amyl acetate or higher molecular weight solvent is effective.The ratio of glass to vehicle and binder will depend to a large extenton the manner of application, the viscosity of the suspension beingadjusted to provide the desired thickness and coverage of powder on thesealing surface.

The suspension may be applied in various manners. Screen printing orspraying are desirable for the ceramic semi-conductor packaging parts.For cathode ray tubes, the suspension may be applied by feeding througha constricted tubular type of reservoir to form a ring or strip justcovering the sealing surface, or dipping the part into a pool ofsuspended sealing glass mixture.

Where the coated articles must be handled or stored, the coating ispreglazed, that is fused or at least partially fused prior to the actualsealing operation. In utilizing such preliminary glazing, it isimportant to avoid initiating devitrification since otherwise thesealing material may be sufficiently altered to interfere with theproper seal formation later on. The maximum permissible glazingtemperature will depend then in large measure on the ease with which theglass-zinc mixture recrystallizes. It is generally desirable to employlower glazing temperatures than the sealing temperature which may be ofthe order of 30° to 50°C below the sealing temperature.

During the sealing cycle, once the assembly has been heated to thesealing temperature and the sealing glass caused to wet the sealingsurfaces and flow into the desired seal configuration, the sealing glassis held at its devitrification temperature for a sufficient time for thedesired devitrification to be completed and thereafter cooled to roomtemperature.

The various features of the invention and the benefits which it providesare more fully set forth in the following examples.

EXAMPLE 1

An intimately mixed sealing glass material was produced by milling 80parts by weight glass IV with 20 parts of zinc silicate, both as powderspassing through a standard 300 mesh screen. This mixture was applied tothe surface of alumina ceramic parts. Heating the parts together at430°C for 20 minutes produced a very strong thermally devitrifiedglass-zinc filler material sealing together the parts to form a hermeticcavity inside the ceramic parts. Similarly heating the glass alone or amixture of 80% glass with 20% beta-eucryptite, fused silica or zircon,even at higher temperature for longer time, produced only partialrecrystallization in the glass and the bond was weak and non-hermetic.

EXAMPLE 2

A series of sealing compositions was prepared by intimately mixing glassIII with increasing amounts of zinc zirconium silicate ranging from 5 to30% by weight. Both powders were 300 mesh. Heating at 440°C for 20minutes or at 460°C for 3 minutes induced higher recrystallization withincreased zinc filler content. The sealing strength increased remarkablyabove 15% by weight zinc filler.

EXAMPLE 3

Similarly lead borate glass X was intimately mixed 5 to 30% zinczirconium silicate powders (300 mesh). The mixtures were heated at410°C. A longer time was required to induce maximum recrystallizationthan in example 2. The extent of recrystallization increased clearly asa function of zinc filler concentration and heating time. The glassalone or mixed with beta-eucryptite, silica or zircon showed no tendencyto recrystallize.

EXAMPLE 4

Lead borate glass X was intimately mixed with zinc oxide powder to forma mixture containing 15% by weight of zinc oxide. The mixture was heatedto 400°C. Zinc oxide dissolved into the glass at a rapid rate causing itto devitrify.

EXAMPLE 5

Similarly lead borate glass X was intimately mixed with a series of zinccompounds. 30% by weight of the following 300 mesh powders were added:zinc silicate, zinc aluminate, zinc zirconate, zinc stannate,recrystallized zinc borosilicate glass powders. The mixtures were heatedto 400°C. Recrystallization of the glass was less rapid than in example4 but became complete. Mechanical strength of the glass-zinc fillermixtures increased rapidly with zinc filler concentrations.

EXAMPLE 6

An intimately mixed sealing glass material was produced by milling 82parts glass VI with 18 parts of zinc zirconium spinel (zinc zirconiumalumina silicate - ZnO(Al₂ O₃)₀.855 (SiO₂)₁.43 (ZrO₂)₁.35) both aspowders passing through a standard 300 mesh screen. 0.5 part of inertblack colored stain was added. The mixture was applied to the surface ofalumina ceramic parts (CerDip). The parts were preglazed at 390°C. Aprepunched metal lead frame was inserted in the melted base part and thepackage sealed at 430°C for 15 minutes. Very tight and strong seals wereobtained. These semi-conductor packages were subjected to 30 cyclesliquid-to-liquid thermal shocks (MIL-STD-883, method 1011, condition C)without affecting their hermeticity.

EXAMPLE 7

An intimately mixed sealing glass material was produced by milling 85parts by weight glass XI with 10 parts zinc silicate and 5 parts zincaluminate (300 mesh powders). The mixture was applied on cathode raytube parts. The parts were sealed at 410°C. A strong, hermetic seal wasproduced.

It is readily apparent from these examples that lead borate and zincpoor lead-zinc-borate glasses can be used to advantage by the additionof zinc containing fillers. The fluidity and the recrystallization rateof these semi-stable sealing glass materials can be controlled by theproper selection of a glass from Table 2 and of zinc containing filleror combination of fillers. Thus, the so-called working properties andworking range of these sealing materials can be adjusted over widelimits according to the particular application.

It should be noted that all of the glasses shown in Table 2 have a smallcontent of SiO₂. The presence of SiO₂ in amount at least about 0.5% byweight of the glass-filler mixtures is highly desirable since the SiO₂slows crystallization of the glass and so permits better control inproducing a proper seal. The selection of the solder glass and fillershould be made with a view to producing a mixture having an SiO₂ contentof at least 0.5% by weight.

It should be noted also that lead-borate glasses (glasses VIII, IX and Xin Table 2) and lead-zinc-borate glasses (the other glasses in Table 2)may contain minor proportions of other oxides as shown which make minormodifications of the glass properties. The terms lead-borate glass andlead-zinc-borate glass are used in the sense that these very minorproportions of modifying oxides may be present.

We claim:
 1. A sealing glass composition comprising a mixture of afinely divided solder sealing glass selected from the group consistingof lead boron glasses and lead zinc boron glasses in which the zincoxide:lead oxide mol ratio is below 1:2 and a finely divided oxygencontaining zinc material selected from the group consisting of zincsilicate, zinc zirconium silicate, zinc oxide, zinc aluminate, zinczirconate, zinc stannate, zinc zirconium aluminum silicate and mixturesthereof, the zinc material being present in amount in the range 3 to 30%by weight of the total mixture.
 2. A sealing composition comprising amixture of a finely divided solder sealing glass selected from the groupconsisting of lead boron glasses and lead zinc boron glasses in whichthe zinc oxide:lead oxide mol ratio is below 1:2 and a finely dividedoxygen containing inorganic zinc material, selected from the groupconsisting of zinc silicate, zinc zirconium silicate, zinc oxide, zincaluminate, zinc zirconate, zine stannate, zinc zirconium aluminumsilicate and mixtures thereof being present in an amount less than 30%of the total mixture but sufficient to give the total composition a zincoxide:lead oxide ratio of at least 1:2.
 3. Composition as defined inclaim 1 wherein silicon dioxide is present as such or as a minorcomponent of the glass or zinc compound in amount constituting at least0.5 percent by weight of the total mixture.
 4. Composition as defined inclaim 1 wherein the zinc material is zinc zirconium aluminum silicate.5. A sealing glass composition comprising a mixture of finely dividedsolder glass containing 75-90% by weight lead oxide, 10-25% by weightboron oxide and 0-13% by weight of zinc oxide and a finely dividedoxygen containing zinc material selected from the group consisting ofzinc silicate, zinc zirconium silicate, zinc oxide, zinc aluminate, zinczirconate, zinc stannate, zinc zirconium aluminum silicate and mixturesthereof, the zinc material constituting 3-30% by weight of the mixture.